1. Field of the Invention
The present invention relates to radar systems. More specifically, the present invention relates to digital waveform generation and receive beam formation in a multiple beam electronic scan antenna array radar system.
2. Description of the Related Art
Certain radar applications use multiple simultaneous transmit and receive beams to enhance their search rate capability. Using multiple simultaneous beams helps maintain situational awareness and allows advanced processing techniques such as clutter cancellation and improved anti-jam capabilities.
Multiple simultaneous beams can be generated using an electronically scanned antenna array (ESA). An ESA is an array of several individual antenna elements which work together to form a desired overall antenna pattern. Each antenna element radiates a common signal altered in phase. The direction of the antenna beam can be controlled by varying the phase of the signals driving the individual antenna elements. Thus, it is required that the signal along each path between a signal source and the antenna elements have a controlled phase.
Conventionally, ESA use analog antenna manifolds to distribute outgoing signals with the correct phase adjustments to the antenna elements in the transmit mode, and to coherently combine antenna element signal components in the receive mode. Such manifolds include complex circuits such as feed antennas, feed distribution networks, and corporate feeds. These manifolds can get very large and heavy in proportion to the number of elements in the antenna array.
The Butler matrix has been used to feed an antenna array for generating multiple simultaneous beams. The Butler matrix, however, suffers from insufficient bandwidth with respect to current more demanding applications, as well as being an inherently complex and heavy piece of equipment.
Hence, a need exists in the art for a system or method for efficient waveform generation for a multi-beam electronically scanned antenna array that offers wide bandwidth, low cost, and low weight.
The need in the art is addressed by the efficient digital wideband waveform generation and signal processing design for electronically scanned antenna array (ESA) radar systems of the present invention.
On transmit, a digital waveform generator produces digital forms of the signals to be transmitted by the individual elements of the antenna array. A different signal is produced for each element. Each of these digital signals is passed through a digital to analog converter (D/A) and a radio frequency (RF) filter at each antenna element. The resulting analog signals are each amplified by a power amplifier and then sent through a wideband circulator to an antenna element.
The digital waveform generator first produces a series of waveform phase angles corresponding to the phase of the waveform to be transmitted. These are common to all the antenna elements. Then, for each antenna element, the waveform phase angles are adjusted to steer the beam by adding a time delay and a phase adjustment. A digital signal is then produced for each antenna element by applying a cosine function to the adjusted phase angles. In the illustrative embodiment, this is accomplished using a cosine look-up table. Each digital cosine wave is then loaded into a serial memory buffer and read out to the D/A converters.
On receive, at each antenna element, a wideband linear amplifier and an RF filter provide rejection of out of band signals while providing amplification and signal conditioning for the following analog to digital converter (A/D). The resulting digital signals (one for each element) are sent through digital frequency channelizers. Like frequency channels from each antenna element are then combined to form multiple digital simultaneous beams. As in the transmit chain, the beam formation can be done with a combination of time delays and phase adjustments.